Matt's RV-7 Project

Not much work on the airplane this weekend, and even less that was worth photographing. I spent several hours sanding the edges of the canopy with increasingly finer grades of sandpaper, starting with 120 grit and working my way down to 600. Then I broke out this buffing wheel and plexiglass polishing compound that I'd previously ordered from Ridout Plastics. (Nice people to deal with on the phone, but they really screw you on shipping – $25 in shipping and handling fees for ten dollars worth of parts?!?)

The special "plexiglass polishing wheel" turned out to just be a normal garden-variety soft cotton buffing wheel. The polishing compound, however, was interesting – instead of the liquid goo I'm used to seeing for a buffing/polishing application, it was almost like chalk in composition. To get it where it needs to be you peel back the cardboard tube and run the wheel across the chalky stuff until it picks enough up.

It certainly did the trick – the edges of the canopy and windshield are now as smooth as the edges of a pint glass. Excellent.

I think I may have finally seen the last of the most obnoxious plexiglass work – or at least I don't think I will need to do much more cutting or sanding. (and I also have enough magic polishing stuff to do another dozen airplanes)

The next tasks will be the canopy latch handle and the rear latch blocks, but I have other stuff I have to do first.

In order to fit the windshield to the fuselage, I clecoed on the forward top skin and bolted the rollbar brace in place:

Here's the windshield on the fuselage before any trimming was done. The contour along the front doesn't match the fuselage all that well.

Here it is about halfway through the trimming process – getting better.

There isn't a lot of excess material to work with at the rear corners – actually the glass just barely overlaps the side skin. In retrospect, I wish I had waited to trim the lower edges of the canopy until after making the big cut, but I was blindly following the plans and didn't know any better.

Much trimming and belt-sanding later, I got the front contour to match the fuselage shape pretty well. Once the windshield is fastened down, there will be essentially zero gap here.

There is gap of about a quarter inch on both sides of the windshield where it turns aft towards the lower corners. The fasteners will pull this in somewhat, but I'll still have to cover it up with the windshield fairing. If I'd had more material to play with I think I could have made this come out better, but overall it's not too bad.

I duct taped the windshield to the rollbar in order to measure the height difference between the windshield and canopy:

The apex of the canopy was about 3/16" higher than the windshield. To fix this you can either raise the windshield with shims, or lower the canopy. I chose the latter because of the afforementioned issue at the aft corners.

The canopy is lowered by whacking a piece off both ends of the forward bow tube:

With the canopy back on the rollers, there's now a smooth transition between the top of the windshield and the top of the canopy:

With the height properly set, and everything clamped straight and true and triple-checked, I drilled the holes for the screws that attach the rollers to the canopy frame:

The canopy rolls very easily, thus making the hours of fitting and measuring that led up to this point all worth it.

The next job was to drill the windshield to the rollbar, so I started with the same masking tape setup I used to drill the canopy to the slider frame:

Here is it after drilling. Because of the angle the windshield makes with the rollbar, the actual centerline where the holes need to go is not where you'd imagine it would be just by looking at it – it's important to use the tape method to get everything square.

The glass overlaps the side skin by about 3/16" on either side. This will all be hidden by a fairing later.

In the photo below you're standing on the right side of the fuselage looking forward towards the firewall. The rollbar and slider frame contours don't quite match, so the windshield bulges out from the canopy by about 1/8" at the 10 and 2 o'clock positions. This will also be covered up by a fairing. I'd be worried if the canopy was sitting proud of the windshield because that would have made the fairing difficult, but this way should be no big feal.

I still need to do at least one more round of trimming to finish off the aft edge of the windshield and finalize the gap between the canopy and the side frame tubes, but I'm hot and tired of being covered with grit.

Update: I couldn't resist spending another hour grinding away and making a mess. I got the canopy trimmed to its final size (I think) and I cleaned up the aft edge of the windshield. Of course I then had to take another shower to wash off all the grit. My next airplane will have a solid aluminum canopy and a TV screen to let me see out.

Mary left me this message on my whiteboard. This is probably good advice for someone working on an RV canopy.

When the plexiglass canopy bubble is clamped to the steel frame, it pulls the sides of the frame outward a surprisingly large amount. If you have previously bent the frame so it rolls perfectly in the tracks, this screws up your alignment and makes the canopy almost impossible to roll. Needless to say, this is undesirable on the finished aircraft. The plans make a big deal about not bending the canopy frame once the plexiglass is drilled to the frame… then they turn around and say to drill the canopy to the frame, then bend the frame so it fits the fuselage again. Don't do this! It's a trap! Once you drill the holes, you are locked in forever. The first statement from the plans is correct – any bending of the frame after the holes are drilled will put stress on the canopy and it will eventually crack.

What you really need to do is first bend the right amount of preload into the frame, so the canopy rolls smoothly with the plexiglass clamped to the frame. Only after the canopy rolls properly with the plexiglass in place should you drill the holes. I re-bent the canopy frame to make its resting size about 1/2" narrower than it was previously, which makes it squeal horribly and resist mightily if you try to roll the frame by itself. However, when the canopy is clamped on it gets wider and the friction on the rollers goes away again.

After doing the above, I put two layers of masking tape on all surfaces where the plexiglass comes in contact with the canopy frame, and marked hole locations every 2" per the plans.

Back on the frame goes the canopy, secured by eleventy billion clamps.

Along the sides where the plexiglass will be secured by the side skirts, I clamped some strips of wood to the frame to keep the bubble from bulging outward.

There are a two conflicting schools of thought on what kind of drill bit to use for the initial holes in the canopy and frame. The plans state that in general you should use special plexiglass bits for enlarging existing holes in plexiglass (because a regular split point bit can grab and crack the material) but they don't specifically say what kind of bit to use to make the pilot holes in the first place. Since a plexiglass bit basically works by melting the material you're drilling, it obviously won't make a hole in steel, which is problematic. I emalled Van's to ask for clarification and was told:

You can drill through the plexi with the plastic drill far enough that it starts a hole in the frame, then remove the plexi and finish the hole with a regular drill.

That sounds like it would involve a lot of removing and replacing the canopy, which doesn't seem very fun. A second method advocated by some folks is to make the first holes with a regular #40 bit, which is what ended up doing. I used a pretty dull old bit just for luck, and turned it fast with almost no pressure so it would sort of melt its way through. I also put a bunch of duct tape over the drill stop to avoid marking the glass when the drill broke through the steel.

Oh boy, we're committed now:

Note in the above photo how a centerline magically appears where the canopy compresses the tape. You still need to mark an X on the outside of the glass before drilling each hole, because there's a pronounced parallax effect that could ruin your day if you're not careful.

I drilled all the holes in the center spine, starting in the middle and alternating forwards and aft. The tape trick made it pretty easy to put all the holes exactly on the point where the glass is tangent to the frame.

At the most forward and aft holes, the shape of the canopy and frame were different enough to require some small shims. I used some nylon washers I had laying around. It's important to shim every hole if/as required, rather than trying to force the plexiglass to follow a shape it doesn't want to. Less stress on the canopy equals less chance of cracking.

Here's the canopy with all the holes drilled and clecoed to the frame. You can roll it open and closed with one finger – victory!

Once the position of the canopy on the frame was locked in, I used the belt sander to even up the forward lip. It's now a fairly uniform 1/16" forward of the front bow across the entire length. I smoothed this up again with 400 grit later.

Then I took the canopy off the frame, and back-drilled the C-653 cover strip through the holes in the plexiglass:

Finally, I enlarged all the holes in the plexiglass to 1/8" with the plexiglass drill bit, and deburred everything. The holes in the frame I left at #40 for now, so I can leave the canopy clecoed to the frame without worrying about the clecoes putting stress on the holes in the plexiglass. Eventually the frame holes will be drilled up to #30 for blind rivets, and the holes in the canopy will go up to 5/32". The idea is to leave room in the holes for thermal expansion all around the fasteners, again with an eye to preventing cracks.

I clamped the canopy to the frame again in order to mark the final trim lines along the side rails. But first I had to go to the hardware store again and pick up even more clamps, if you can believe it. I ended up using them all, too.

I also marked a line for further trimming along the aft edge. This will give me 1/2" of overhang, or 3/4" of edge distance from the fasteners – right in the middle of the suggested range.

Here it is after trimming. I know, it doesn't look any different, but it took a lot of itchy, nasty, obnoxious work to get from the first photo to this one.

I went over all four edges with the belt sander again, then 220 and 400 grit paper. I also made an extra effort to knock the sharp corners off all the cut edges, which I slacked on yesterday. The edges aren't glass-smooth yet but they're okay for now.

Back on the fuselage and all clamped down. Everything seems to have pulled into place nicely. The sides of the bubble are now 1/4" above the canopy frame side rails, which will make room for some aluminum bits that attach the plexiglass to the frame.

Lots of canopy scraps in the trash… I hope all these pieces were supposed to be removed!

I need to come back with the belt sander and make the visible forward lip of the canopy arrow-straight, but I'll wait to do that till after the plexiglass is drilled to the frame and nothing is going shift around.